Add table function mergeTreeIndex

[CurtizJ]

Changelog category (leave one):

• New Feature

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Added table function mergeTreeIndex. It represents the contents of index and marks files of MergeTree tables. It can be used for introspection. Syntax: mergeTreeIndex(database, table, [with_marks = true]) where database.table is an existing table with MergeTree engine.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

[robot-clickhouse-ci-1]

This is an automated comment for commit 5330824 with description of existing statuses. It's updated for the latest CI running

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[alexey-milovidov]

And row count for adaptive granularity?

[CurtizJ]

It's already added (column rows_in_granule)

[CurtizJ]

Test failures are unrelated.

[tavplubix]

Test failures are unrelated.

Stress test (asan) — Found signal in gdb.log and Upgrade check (msan) — Possible deadlock are still quite interesting, are they already known?

However, there's another critical issue introduced by this PR, and I failed to fix it quickly, so I have to revert, sorry :(